Positive photoresist compositions in use generally comprise an alkali-soluble resin and a naphthoquinonediazide compound as a photosensitive substance. For example, photoresist compositions comprising "a combination of a phenolic novolak resin and a naphthoquinonediazide substitution compound" are described in, e.g., U.S. Pat. Nos. 3,666,473, 4,115,128, and 4,173,470. Further, an example of the most typical composition comprising "a combination of a cresol-formaldehyde novolak resin and a trihydroxybenzophenone-1,2-naphthoquinonediazidesulfonic acid ester" is described in L. F. Thompson, "Introduction to Microlithography" (ACS Press, No. 2, 19, pp. 112-121).
In such a positive photoresist consisting basically of a novolak resin and a quinonediazide compound, the novolak resin imparts high plasma etching resistance and the naphthoquinonediazide compound functions as a dissolution inhibiting agent. The naphthoquinonediazide has the property of generating a carboxylic acid upon light irradiation to thereby lose its dissolution inhibiting ability and enhance the alkali solubility of the novolak resin.
Many positive photoresists comprising a novolak resin and a photosensitive naphthoquinonediazide compound have been developed and put to practical use so far from the above-described standpoint. These photoresists have produced satisfactory results in the formation of resist patterns having line widths ranging about from 0.8 to 2 .mu.m.
However, the degree of integration in integrated circuits is increasing more and more, and it has become necessary to form an ultrafine pattern having a line width of 0.5 .mu.m or smaller in the production of semiconductor substrates for VLSIs and the like. For attaining the necessary resolving power, the wavelengths of the light sources used for photolithography are decreasing more and more and, as a result, use of far ultraviolet rays and excimer laser beams (XeCl, KrF, ArF, etc.) has come to be investigated.
The prior art resists comprising a novolak and a naphthoquinonediazide compound are unsuitable for use in pattern formation by lithography using far ultraviolet rays or excimer laser beams, because the novolak and the naphthoquinonediazide exhibit intense absorption in the far ultraviolet region to render the light less apt to reach the resist bottom. Thus, the resist has low sensitivity to give only a tapered pattern.
One means for eliminating the above problem is the chemically amplified resist composition described in, e.g., U.S. Pat. No. 4,491,628 and European Patent 249,139. A chemically amplified positive resist composition is a pattern-forming material in which an acid generates in exposed areas upon irradiation with a radiation such as far ultraviolet rays and this acid catalyzes a reaction that makes the areas irradiated with the actinic rays and the unirradiated areas which are different in solubility in a developing solution to thereby form a pattern on a substrate.
Examples thereof include combinations of a compound which generates an acid upon photodecomposition with an acetal or O,N-acetal compound (see JP-A-48-89003; the term "JP-A" as used herein means an "unexamined published Japanese patent application"), with an orthoester or amidoacetal compound (see JP-A-51-120714), with a polymer having acetal or ketal groups in the backbone (see JP-A-53-133429), with an enol ether compound (see JP-A-55-12995), with an N-acyliminocarbonic acid compound (see JP-A-55-126236), with a polymer having orthoester groups in the backbone (see JP-A-56-17345), with a tertiary alkyl ester compound (see JP-A-60-3625), with a silyl ester compound (see JP-A-60-10247), and with a silyl ether compound (see JP-A-60-37549 and JP-A-60-121446). These combinations exhibit high photosensitivity since they have a quantum efficiency exceeding 1 in principle.
Another means for eliminating the problem described hereinabove is a system which is stable over long at room temperature but decomposes upon heating in the presence of an acid to become alkali-soluble. Examples thereof include systems comprising a combination of a compound which generates an acid upon exposure to light with an ester having a tertiary or secondary carbon (e.g., t-butyl or 2-cyclohexenyl) or with a carbonic ester compound, as described in, e.g., JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642; Polym. Eng. Sce., Vol. 23, p. 1012 (1983); ACS. Sym., Vol. 242, p. 11 (1984); Semiconductor World, p. 91 (November 1987); Macromolecules, Vol. 21, p. 1475 (1988); and SPIE, Vol. 920, p. 42 (1988). Since these systems also have high sensitivity and exhibit reduced absorption in the deep UV region as compared with the naphthoquinonediazide/novolak resin systems, they can be effective systems for coping with the wavelength reduction in light sources.
The chemically amplified positive resists described above are roughly divided into two groups: three-component systems comprising an alkali-soluble resin, a compound which generates an acid upon exposure to a radiation (photo-acid generator), and a dissolution inhibitive compound for the alkali-soluble resin which has acid-decomposable groups; and two-component systems comprising a resin which decomposes upon reaction with an acid to become alkali-soluble and a photo-acid generator.
In these two-component or three-component, chemically amplified positive resists, the photo-acid generator is caused to generate an acid by exposure to light and the resists are then heat-treated and developed in the presence of the acid to obtain a resist pattern.
Known photo-acid generators for use in the above-described chemically amplified positive resists include N-imidosulfonates, N-oximesulfonates, o-nitrobenzylsulfonates, and pyrogallol trismethanesulfonate. Typical compounds which have been used as photo-acid generators having a high photodecomposition efficiency and excellent image-forming properties are the sulfonium and iodonium salts of perfluorinated Lewis acids, e.g., PF.sub.6.sup.-, AsF.sub.6.sup.-, and SbF.sub.6.sup.-, described in, e.g., JP-A-59-45439 and Polym. Eng. Sci., 23, 1012 (1983).
However, these prior art photo-acid generators, when used in resist materials for semiconductors, have a problem that the counter anions of the photo-acid generators cause pollution by phosphorus, arsenic, antimony, etc.
Used as a sulfonium or iodonium compound free from the pollution is the salt described in, e.g., JP-A-63-27829, JP-A-2-25850, JP-A-2-150848, JP-A-5-134414, and JP-A-5-232705, in which the counter anion is a trifluoromethanesulfonate anion.
It should, however, be noted that this prior art composition has a problem that since trifluoromethanesulfonic acid, which generates upon exposure to light, diffuses relatively rapidly in the resist film, the line width of the resist pattern which is being produced becomes narrower with the lapse of time from exposure to light to heat treatment or the resist pattern comes to have a T-top surface.
Although use of a toluenesulfonate anion as another counter anion for sulfonium or iodonium is described in, e.g., JP-A-2-25850, JP-A-2-150848, JP-A-6-43653, and JP-A-6-123972, this salt has a problem that since it has insufficient solubility in ordinary resist solvents, the addition amount thereof is limited, resulting in insufficient sensitivity.
Further, the use of resins having a sulfonium group in the polymer backbone or in side chains thereof is disclosed, e.g., in JP-A-2-302758, JP-A-4-230645, and U.S. Pat. Nos. 5,130,392 and 5,260,410. These resins having a sulfonium group have the same problem in diffusibility in a resist film as that of the sulfonium or iodonium compound having a trifluoromethanesulfonate anion as the counter anion, since an acid itself they generate upon exposure to light is a low-molecular monomer.